Electrolyser cell hardware comprising only a polymer presents difficulties in pressure retention, dimensional stability, and resistance to hoop stress. It is desirable to operate an electrolyser at high pressures, self-pressurising it using the proton pump effect to save energy in compression to the product gas. Typically, the pressure associated with thresholds enabling a reduction in the number of compressor stages and associated costs are 15, 30, 80 bar gauge.
At 15 bar and 60° C., for a given electrolyser cell made from polymer alone, having an internal diameter of 230 mm and an outer diameter of 320 mm, with a Polyphenylene Sulphide (PPS) glass-filled material, the average hoop stresses would be in the region of 4.3 MPa. This is easily calculated with the hoop stress formula, known to those familiar with the art.
                    σ        =                              (                          d                              2                ⁢                                                                  ⁢                t                                      )                    ⁢                      P            eff                                              (        1        )                σ: hoop stress    d: mean diameter=275mm    t: ring wall thickness=45mm    Peff: effective pressure Peff=Pinternal−Patm=1.5-0.1=1.4 MPa
                    σ        =                                            (                              275                90                            )                        ⁢                                                  ⁢            1.4                    =                      4.3            ⁢                                                  ⁢            M            ⁢                                                  ⁢            Pa                                              (        2        )            
200 MPa is the ultimate strength of the material envisaged in the preferred embodiment (Polyphenylene Sulphide). High-pressure electrolysers are not expected to retain pressure continuously. Depending on the percentage utilisation and cyclic energy input profile (often a renewable energy input), the electrolyser will be subjected to a corresponding cyclic loading. It will very slowly depressurise after gas generation is stopped, whilst accumulated gas in adjoining store will be prevented from flowing back using non-return and pressure control valves.
Therefore to test a cell assembly according to these high-pressure conditions, a test consisting of a series of pressure cycles from 0 to a pressure including a factor of safety should be carried out. The test pressure should be 1.43 times the value of the relief pressure setting, typically set at 18 bar for 15 bar working pressure. Therefore the pressure the cell is subjected to is 26 bar G; this would qualify a pressure retention assembly to 15 bar working pressure. 5000 cycles correspond to approximately 4.5 years running, assuming 3 full pressure cycles per day. At 15 bar the stress levels are modest with respect to the geometry and materials considered.
Since higher pressures are desirable (30 and 80 bar), the above calculation example shows that it is imperative to strengthen the parts. This poses greater challenges.
One construction commonly encountered is a full steel ring construction. This is expensive when stainless is used, if stainless steel can be used. If titanium has to be used instead, it is prohibitive.
An alternative to the latter is a secondary containment vessel used as an external pressure jacket, to counter act hoop stresses from the outside. This is satisfactory for discrete installation (research tools), but industrially, it is not conducive to manufacturability, nor is cost effective.
It may also be possible to apply a significant axial load on the faces of the cell plate to increase its stiffness matrix. This has little to no effect to counter the action resulting from internal pressure since the load directions are not opposing each other. Replacing the polymer by another tougher polymer is done invariably at the expense of elongation; the latter being important to prevent catastrophic fracture. Elongation is itself negative as it affects the membrane to cell internal diameter fit.